This invention relates to superconducting films, and a method for their production. More particularly, this invention relates to superconducting thin films of niobium carbonitride of high quality (i.e., high transition temperature T.sub.c, high critical current J.sub.c, low RF losses, etc).
Superconductors are electrical conducting elements capable of substaining large currents without electrical loss under cryogenic-temperature operating conditions. The precise amount of current that can be carried without loss is a complex function of the material's temperature, magnetic field, chemical composition, and metallurgical history.
Superconducting films have a wide variety of applications, some of which are: conductors for superconducting magnetic or power transmission lines; high Q, low loss cavities required for ultra-stable frequency standards; utlrasensitive magnetometry; and digital circuitry applications.
It is well known that certain metals, alloys and compounds go through a superconducting transition into a state in which electrical resistance virtually ceases to exist at temperatures approaching absolute zero. The temperature at which such a material enters this superconducting state is known as the transition temperature, T.sub.c. As long as the temperature of the material remains below its particular T.sub.c, a large current will flow, unless the magnetic field rises to a critical level, H.sub.c. This critical field, H.sub.c, is itself a function of temperature, increasing in value as the temperature dips below T.sub.c and approaches absolute zero.
It is desirable that T.sub.c be as high as possible, just as it is desirable that H.sub.c be high. The reasons for this are simple; a higher T.sub.c allows a much higher operating condition without disrupting performance, thus leading to reduced refrigeration costs together with higher performance. A higher H.sub.c allows more current to flow, as well as enabling larger magnetic fields to be used, thus leading to greater performance capabilities. In addition, the critical current density, J.sub.c, which is the maxium density of current flowing through the material at a given operating temperature and magnetic field which will cause the superconductor to go from the superconducting to the normal state, is a function of H.sub.c and T.sub.c. Much emphasis in the field of superconductivity has centered around finding materials that exhibit high T.sub.c, H.sub.c, and J.sub.c.
The superconducting properties of niobium carbonitride are well known; films of NbCN exhibiting a critical temperature as high as 18.degree. K. and a H.sub.c of more than 25 T have been made. Several techniques of preparing NbCN films are known; chemical vapor deposition, sintering pressed compacts of NbN and NbC powders, and most recently, reactive sputtering of niobium in a methane-argon vacuum atmosphere. However, these techniques have been unable to avoid the contamination with unwanted elements that lowers performance qualities. Thus, the development of a homogeneous, thin niobium carbonitride film of high purity presents a new and difficult problem.
The prior art has made several attempts at producing thin superconducting films with optimum properties. U.S. Pat. No. 3,912,612 Gavaler et al, uses a D.C. reactive sputtering techniques to produce thin superconducting films such as NbN. Hydrocarbon gases, such as methane, are added to the vacuum chamber, along with nitrogen and argon, and react with the sputtered niobium to produce NbN, and other films. Contamination from even the trace amounts of residue gases which are present, even in extremely high vacuums, presents a serious problem which can seriously damage the superconductive properties of the film. U.S. Pat. Nos. 3,951,870 and 3,912,461 both prepare NbCN films; the 3,951,870 patent produces the material in the form of continous fine diameter multifilament yarn instead of thin films, and the 3,912,461 patent is concerned with providing very hard, durable and abrasion resistant coatings. An article by Brunet et al, High Field Critical Current Densities and Resistive Transition on NbCN Superconducting Films, Vol. 19, No. 2, in the February 1979 "Cyrogenics", studied the influence of the carbon density on the critical current densities (J.sub.c) of thin NbCN films in high magnetic fields. The influence of the carbon-nitrogen ratio on the high field properties, specifically J.sub.c, was also investigated. However, no system has yet been able to optimize thin NbCN superconducting films by substantially avoiding the contamination exhibited by earlier films.